The production of open cell flexible polyurethane foam employs a variety of additives and each one of them plays a role in determining the final characteristics and physical properties of the product. Although these additives represent a small percentage in the overall formulation and their emissions are expected to be relatively low, the increasing demand for low volatile organic contents (VOC's) in finished products has placed additional requirements on additives to achieve these lower emissions while maintaining foam performance.
It is well known that conventional manufacturing procedures to make polyurethane foams use additives that are emissive. One of the strategies used to reduce emissions from foam is based on introducing functional groups on tertiary amine catalysts able to react with isocyanate. Using this approach, the tertiary amine catalysts would remain covalently bonded to the polyurethane polymer preventing its release into the environment. This approach can have some limitations because: a) the functionalized tertiary amine can react with isocyanate prematurely causing undesired side effects such as polymer chain termination which would result in poor physical properties, b) excessive cell opening or foam collapse or excessive cross linking which can result in extensive shrinkage and poor dimensional stability, c) poor physical properties particularly when measured after accelerated thermal humid ageing due to the catalyst remaining in contact with the polyurethane material causing its degradation, d) poor physical properties because the amine catalyst gets immobilized prematurely in the polyurethane polymer being unable to fully finish the curing process, and e) relatively high use levels of catalysts are required due to amine irreversible immobilization in the polyurethane polymer.
Another alternative approach to reduce odor and emissions is based on utilizing materials with increasing molecular weight and/or polarity. However, the limitation of this approach is the required higher use level of the catalyst due to the lower catalytic efficiency due to molecular mobility.
Products such as dimethylaminopropyl urea, bis(dimethylaminopropyl) urea, bis(dimethylaminopropyl) amine and N,N-bis(dimethylaminopropyl)-N-(2-hydroxypropyl) amine can provide acceptable ambient physical properties as compared to industry standards whereas most conventional reactive catalysts cannot always achieve today's consumer and manufacturer requirements. Using these catalysts can reduce significantly the overall emissions from foam. However, ppm levels of amine catalysts can still be detected in finished articles when emissions are measured according to VDA 278 detection method.
One key feature required for the isocyanate reactive tertiary amine catalyst relates to its ability to form a thermally stable covalent bond with the growing polyurethane polymer. The covalent bond should be stable enough to retain the amine catalyst in the polyurethane polymer when foam sample is heated and emissions are removed from the heated chamber by the constant flow of inert gas. Currently, there are a wide variety of functionalized amine polyurethane catalysts capable of reacting with isocyante during the polymerization process. However, in some cases foam produced with some of these reactive catalysts can still have amine emissions because the covalent chemical bonds that holds the amine catalysts into the polyurethane polymer are not sufficiently stable at the temperature of the test. Also, in some other cases foam produced with reactive amine catalysts do not have amine emissions because the covalent chemical bonds that holds the amine catalysts into the polyurethane polymer are sufficiently stable at the temperature of the test but the amine is too reactive towards the isocyanate group leaving the catalyst immobilized early on in the polymerization process with the net result that the finish polyurethane product has poor physical properties or it might partially meet certain physical properties while failing others.
Without wishing to be bound by any theory or explanation, it is believed that such emissions could result either in the release of the amine catalysts from the polyurethane polymer or in the release of by-products and chemical fragments from the thermal decomposition of the amine-polymer adduct.
In addition to thermal stability, these catalysts preferably form hydrolytically stable covalent bonds under a wide variety of conditions and pHs. Hydrolytic stability of the chemical bond between the tertiary amine and the polyurethane polymer plays an important role in applications where polyurethane foam is in contact with textiles that can be exposed to moisture and/or water or in applications where foam can directly be exposed to water while in contact with skin. If the hydrolytic stability of the chemical bond between the polymer and the tertiary amine is not sufficient then tertiary amine catalyst can leach from the polyurethane polymer and may allow amines to directly contact skin leading to skin irritation or skin sensitization.
Finally, thermal stability and catalyst immobilization at lower isocyanate index is an additional performance requirement. In addition to thermal stability at typical indexes such as 90-115 new catalyst need to be able to form covalent bonds with polyurethane polymer that have thermal stability and no emissions at indexes as low as 65 and typically higher than 60. This is a requirement that is difficult to meet because at low isocyanate index there is not sufficient NCO groups able to react with all OH groups from polyols and water so the new amine additive needs to be able to provide simultaneously sufficient catalytic activity to provide good quality foam and effectively compete with OH groups from polyols and water to become part of the polyurethane polymer and be retained in the polymer once the polymerization process is completed. U.S. Pat. No. 5,859,079 disclosed a polyurethane catalyst composition that comprises N,N′-bis(3-dimethylaminopropyl)urea and 3-dimethylaminopropylurea. However when a cured polymer is heated to temperatures as high as 120° C. amine emissions occur. In addition, water contacting foam produced using this catalyst can have an increased alkalinity. U.S. Pat. No. 6,858,654 discloses a catalyst composition for promoting the polyurethane forming reaction which includes a gelling catalysts and a blowing catalyst. The gelling catalyst are selected from tertiary aminoalkyl substituted primary or secondary amines and the blowing catalysts are selected from bis(aminoalkyl)ethers comprising alkanol moieties, primary amine moieties, or ureido moieties derived from such primary amine moieties. Foams produced with this catalyst are able to provide finished products with no amine emissions, however they cannot meet all physical property requirements.
U.S. Pat. No. 4,101,470 discloses compounds having a OH group able to react and form a covalent bond with isocyanate. An example of such a compound can be obtained when reacting bis(dimethylaminopropyl)amine with propylene oxide to yield bis(3-dimethylaminopropyl)(2-hydroxypropyl)amine. One limitation of the composition is lack of thermal stability of the chemical bond as illustrated in the examples shown in U.S. Pat. No. 6,858,654 where 190 ppm decomposition products from bis-(3-dimethylaminopropyl)(2-hydroxypropyl)amine is observed when foam is heated to 120° C. during testing according to VDA278 emissions test method.
U.S. Pat. No. 4,049,591 claims a method for producing a polyurethane foam which comprises reacting an organic polyisocyanate with an organic polyester polyol or polyether polyol in the presence of a catalytic amount of a compound having a general formula [R″R″N—(CH2)3-]2NCH2CHRY where R″ is a lower alkyl, R is hydrogen or lower alkyl and Y is selected from the groups consisting of CN, CONH2, CO2R′, CONR2′ and COR′ where R′ is independently H, lower alkyl or aryl. Limitations of these compounds includes emissions due to the lack of functionality able to react with NCO or inability to form thermally stable covalent bonds as well as hydrolytic instability.
The disclosure of the previously identified patents is hereby incorporated by reference.
There is a need in this art for foam made with polyurethane catalyst wherein the resultant foam passes emissions tests, One example of an important emission test is called VDA278. This test requires direct desorption (using heat and a flow of inert gas) of a representative mass of sample (PU foam). Volatile and semi-volatile organic compounds are extracted from the sample into the gas stream and are then re-focused onto a secondary trap prior to injection into a GC (MS) for analysis. VDA278 comprises two extraction stages; 1) VOC-analysis: this involves desorbing the sample at 90° C. for 30 minutes to extract volatile organic compounds and analyzing the emissions by GCMS up to a retention time provided by n-C20 aliphatic hydrocarbon standard. This is followed by semi-quantitative analysis of each compound as μg toluene equivalents per gram of sample and 2) FOG-analysis: this involves further desorbing the same sample to 120° C. for 60 minutes to extract semi-volatile organic compounds and analyzing the emissions by GCMS at a retention time interval provided by n-C16 to n-C32 aliphatic hydrocarbon standards. This is also followed by semi-quantitative analysis of each compound as μg n-hexadecane equivalents per gram of sample. More recently the VDA278 analysis was modified as to utilize in VOC-analysis n-C26 aliphatic hydrocarbon standard so as to extend the GCMS retention time range to ensure higher scrutiny of emissions. Similarly, the FOG-analysis was also modified by utilizing n-C14 to n-C32 aliphatic hydrocarbon standards as to expand the window for monitoring emissions. Thus, catalysts that were previously known as non-emissive or able to pass VDA278 do not necessarily pass the new revisions to the method. Thus, there is also a need in this art for catalysts that react with isocyanates and form thermally stable covalent bonds that are able to withstand the testing conditions that reflect extreme environmental conditions. Such a need can become a challenge as the isocyanate index is reduced to low levels (Index as low as 65 however higher than 60) because there is stoichiometrically an insufficient amount of NCO to react with all OH from polyol and water. In some cases, highly reactive amines will pass the emissions tests but fail on providing some key physical property performance while in other cases less reactive amines will fail emissions test while meeting physical properties. In addition, the needed catalysts should be able to form hydrolytically stable covalent chemical bonds to prevent leaching of amine catalyst from the polyurethane article to avoid amine exposure to end users (e.g., when foam gets directly or indirectly in contact with humidity/moisture and heat). Moreover, the covalent bonds between tertiary amine catalysts and polyurethane polymer should be stable under extreme environmental conditions of heat and humidity such that in the event a polyurethane contacts other materials (for example polycarbonate in contact with polyurethane) the other materials are not damaged or deteriorated.